1. Field of the Invention
The present invention relates to a variable capacitance circuit whose electrostatic capacitance can be changed at high speed, a voltage measuring apparatus that is equipped with such variable capacitance circuit and can measure the voltage of a measured body without contacting the measured body, and a power measuring apparatus that uses such voltage measuring apparatus.
2. Description of the Related Art
A variety of voltage measuring apparatuses are known, such as those disclosed by Japanese Laid-Open Patent Publication Nos. H04-305171 and H07-244103.
Out of such voltage measuring apparatuses, according to the conventional technology disclosed by Japanese Laid-Open Patent Publication No. H04-305171, a voltage measuring apparatus (distance-compensating potentiometer) used to measure the charge density of both surfaces of a measured object (a charged body) includes a probe unit, in which a detection electrode and a vibrating body are incorporated, an oscillator, a preamp, an amplifier, a synchronous detector, an integrator, a high-voltage generator, and an impedance matching circuit. With this voltage measuring apparatus, the detection electrode is vibrated using the vibrating body and disposed facing the measured object. When doing so, since the electrostatic capacitance formed between the detection electrode and the measured object changes and therefore the electric field strength between the measured object and the detection electrode changes, an AC voltage corresponding to the electric field strength between the measured object and the 5 detection electrode is generated at the detection electrode. The high-voltage generator generates a DC voltage corresponding to this AC voltage and feeds the DC voltage back to the probe unit. When doing so, the electric field strength between the measured object and the detection electrode becomes zero when the voltage of the probe unit matches the voltage of the charged body. Accordingly, by detecting the DC voltage fed back from the high-voltage generator to the probe unit when the electric field strength between the measured object and the detection electrode becomes zero, that is, when the AC voltage generated in the detection electrode reaches zero volts, the voltage of the measured body is measured.
On the other hand, with the conventional technology disclosed by Japanese Laid-Open Patent Publication No. H07-244103, in the disclosed voltage measuring apparatus (a distance-compensating surface potentiometer), electric charge that is proportional to the potential of the measured object (the “sample”) and is proportional to the electrostatic capacitance between the detection electrode (the “fixed electrode”) and the measured object is generated at the surface of the detection electrode by electric flux lines, out of the electric flux lines emitted from the surface of the measured object, which pass through an input hole and reach the detection electrode. A conductive sector is mechanically moved so as to repeatedly shield and then release the electric flux lines that pass through the input hole to reach the detection electrode. By doing so, charge is repeatedly generated at and removed from the detection electrode, a current that is proportional to the rate of change in the charge flows to a load resistance, and an AC voltage is generated across the load resistance. Although the voltage measuring apparatus disclosed by Japanese Laid-Open Patent Publication No. H07-244103 differs to the voltage measuring apparatus disclosed by Japanese Laid-Open Patent Publication No. H04-305171 described above in the construction for generating the AC voltage, the other parts of the construction are fundamentally the same as in the voltage measuring apparatus disclosed by Japanese Laid-Open Patent Publication No. H04-305171. By reducing the potential difference between the probe case (or the equivalent to the case of the probe unit) and the measured object based on the AC voltage generated across the load resistance, the potential of the measured object is measured in the same way as by the voltage measuring apparatus disclosed by Japanese Laid-Open Patent Publication No. H04-305171.